Skype for Business 2019 "Error : User has insufficient permissions" when logging onto MACP

3 checks you will find:
- SIP enabled
- CsAdministrators
- ManagementOData

There is another one…

Event ID 1310 in Application log warns about an error on /OcsPsws.

The solution is to edit C:\Program Files\Skype for Business Server 2019\Web Components\OcsPsws\Int\Web.config and replace:
<vdirAuth authType="WebTicketOrADFSOnPrem">
with:
<vdirAuth authType="WebTicket">